Exterior RMH for interior heat?

Hello all. New to the site and though I have some older information about Rocket Mass Heaters, my information may be a little dated.

In any event, I live in a very old wood frame farmhouse, with a post-beam foundation that would be well-inadequate for an interior Rocket Mass Heater. I have a dated manufactured fireplace that quite frankly needs to be removed for safety and blows through wood at breakneck speed. It just is not practical for anything other than absolute dire emergencies. I would like to have a rocket mass heater to assist in the heating of the main area of my house, which is about 800 sq. feet, but the only way that seems possible is to build a cob addition on the house and put it there. Has anyone here ever heard of or done a rocket mass heater outside but that captured the heat somehow and vented it into or beneath the house? Any and all ideas on this would be greatly appreciated.

(Having considered many options, I do not have adequate sun for solar heat, though a space heater or two might help a little during a select few hours of the day.)

Much obliged.

I am in the same situation. I am working on an idea.

Thanks very much. A copper coil thermosiphon in the cob might work, but that adds fluid to the system that has to then be converted into warm air somehow. I will keep researching and see what I can come up with. All other thoughts welcome.

can you put it in a under floor situation like a basement? Maybe a system of heat risers like the old gravity furnaces used. At least for a portion of the house. Every bit helps.

Heat mineral oil, circulate it to the inside of your home, then back to the rocket heater so it can make the circuit again.
Oil can attain and carry a much higher temperature than water. Mineral oil is already used in many electric space heaters. The electric element comes on, heats the oil, and the heater radiates that heat. The element comes on periodically to keep the oil hot, which you pay for when your electric bill arrives.
Why not skip the electric heating and use your rocket heater, fireplace, or any wood burning stove to heat the oil, then circulate it to whatever area you want to heat?
You can use this method for subfloor heating, radiator/furnace fan heating for air ducts, or to individual radiator heaters in selected rooms.
The oil can go to over 500 F without bursting into steam and causing excessive pressure, water can not. Oil freezes at -22 F, water at 32 F, meaning oil can be used outside without so much fear of freezing & breaking pipes. (This has big implications for solar batch heaters in cold climates)
I haven't got this quite figured out yet, but circulating the hot oil to heat a thermal mass inside so it radiates that heat for hours should be possible.
In any case, that's my idea. Have fun and please let me know if any of you attempt it. Thanks.

Daniel: There is a crawl-space only, unfortunately.

Bill: That might work with the addition of radiators inside, but since this is an old farmhouse there is no subfloor -- just old hardwood floors. So no floor heating. What about running water or oil into the mass and then have a water to air heat exchanger like an outdoor wood furnace? The warm air then gets routed to the ductwork, like an other outdoor wood furnace, or is the heat just not enough to adequately heat the oil?

Lucy, yes, you have it right. If you plan to heat water, be sure to design it to handle pressure from steam.
No subfloor heating. That's too bad, but you already have a good plan for using the oil to heat the heat exchanger.
Nice to meet you, BTW.

Thanks, Bill. Nice to meet you as well.

Lucy,

I'm not that wild about rocket mass heaters, having built a few of them, but if that's what you want, chop a hole in your floor, support the joists you cut, place stone, or cob, or concrete on the ground in your crawlspace high enough to build your rmh on it. Be sure to insulate that foundation before you build your stove. Now the weight of the thermal mass rests on the earth. If you build the rmh yourself pay close attention to the geometry of burn tube, riser, manifold and exhaust. This stuff is important.

I like furniture that is light, adjustable and portable, so the cobb/bench/thermal mass doesn't interest me. You might want to consider placing your rmh at an elevation that allows the thermal mass to be even with/part of your floor. If you do that you should get the exhaust pipe immediately outside when it exits the mass. And be sure to insulate the mass anyplace it is not in the heated space.

Somebody wrote here that a hydronic system would require changing hot liquid to warm air. No. You want the hydronic system to heat a radiator. It radiates, just like your rmh. Moving air is a bad way to move heat energy.

Good luck

Hi, Dave. Would you explain about not wanting to heat air, please. Isn't that the way most heating furnaces work? Don't they blow air over hot pipes, then through the ductwork and into the home? Not criticizing, just trying to understand your thoughts on this.

Hi Bill,

You're right, many if not most furnaces heat air and blow it into the house through ducts and a return duct takes cooler air back to the furnace. The reason it is so common is that it is a low cost system. It works, but is a terrible way to heat our bodies, which is our goal. (To be fair these systems have an advantage other than cost. The heating and cooling curve is steep, when you want it to be and they convert to air conditioning systems in hot climates quite easily. They can also act as air filters but usually just move pollutants around and around so you get to breathe them more often.)

Heat energy moves through conduction, think of that as the hot handle on your frying pan. Convection, in which warm air rises and cold air sinks, think of the warm air rising to the ceiling in your house, and radiation, think of hot lights, or a wood fire outside in freezing weather when you can feel the heat from the fire on your face. You will often hear people say that "heat rises." No, heated substances rise. Heat is a form of energy that radiates in all directions, even in the absence of air. Radiation is the most efficient way to heat our bodies. In fact, in a home heated with radiation you can set the thermostats lower and be just as comfortable as you would be with a forced air system. Radiant heat does heat objects which then heat the air next to them so the overall effect is a mixture of everything but meanwhile, your body feels the radiant heat whatever the air temperature. Another way to say this: heat can get to your skin in two ways, either as radiation or being carried by air, water, or conduction from a material.

The old style cast iron heaters were quite efficient but because there isn't much thermal mass there, the heating/cooling curve was relatively steep compared to mass heaters, which go back hundreds of years. The mass of material, stone, brick, cobb, absorb huge amounts of heat and then radiate it into the room over a long period. The heating/cooling curve is very flat. (This can be a disadvantage if the weather quickly turns warmer and your mass heater is still pouring out the heat. You can't stop it.) The air in the room does get warmer from the objects that have been warmed by radiation but we are not using the air as a medium to transfer heat.

The best system, in my opinion is the hydronic in- the- floor system. I've built many houses with these including my own and owners always love it. It combines the efficiency of using water as the transfer medium and the floor gives us a huge mass for radiation. Any energy source can be used to heat the water and most modern boilers are extremely efficient. The only disadvantage is cost. They are not cheap up front. Long term it is the only way to go.

A wood fired rocket mass heater with a hydronic loop doesn't make much sense but a rmh with an in-floor exhaust tube does but there are potential problems. Here's one: let's say you build your rmh at a level so you can exhaust it in a cobb or concrete floor. Before you do the floor over it you can test it but it is not performing in the mass you will add to it, so your test is not valid. When you build your floor on it you can only hope it will perform as it should. If it doesn't, what do you do? As a builder I hate things that are buried, that I can't see and fix. That brings us back to the old cast iron radiators with the pipes (insulated of course) running through basements or crawl spaces. Every part of the system is visible and easy to change. Again, as a builder I know that something will go wrong, someplace, where it is least expected.

Sorry to be so long-winded. I hope I answered your question. I remain fascinated by heating, air changes and moisture in houses.

Don't be sorry to write long responses. You explain things well and I appreciate the time you take to do so. Thank you.

I understand what you mean by efficiency. Heating mass is more efficient than hydronic floor heating, but the maintenance advantages of floor heating are worth the sacrifice of efficiency, if I'm understanding you correctly. In both cases, either is more efficient than heating air and blowing it throughout the house.

I've been watching different energy technologies as well, weighing different benefits and drawbacks of each for end users. There are so many factors involved. Economic efficiency, ongoing energy input cost, maintenance costs, performance parameters, initial installation cost, environmental impact, ect...
It's always going to be a balancing act of many factors.
I mean, nuclear would probably be best, but who could afford that at the home level? Not to mention the legality of that. So, nuclear power is out for home production of energy.

What is the maximum heat a hydronic floor heating system can handle without complicating it with special pressure relief valves and upgraded piping?

Is there any to reduce the cost of installation? What is the most common heat source you see out there for this? Did you have any ideas for a better/cheaper heat source?

If you could design the perfect heating/cooling system for a home, what would it be?

Bill,

First, one of your questions is easy. Any time you heat water in containers you must have expansion tanks and pressure relief valves for safety. Not expensive. Now, on to the bigger, and harder questions:

Basically just think of the two major factors in heating. 1. the source of the energy and 2. the delivery system. Under floor heating, rmh, masonry heaters, massive walls, are all mass heaters where we deliver heat energy to a thermal mass that then radiates it back to us over time. Every piece of the system we choose will have positives and negatives so the choice we make depends on a bunch of factors about the site, the building, our age and physical ability, personality etc. We try to put all the pieces of the puzzle together to create the best system for us. This is before we even approach the questions of environmental issues.

The delivery system I like best is in floor. It is efficient, quiet, clean. Sometimes you forget you have a heating system. As I said, the bad part is cost. Another negative, for some people is the floor itself. In floor heat works best in a concrete or gypcrete floor. There are systems of transfer plates that can be attached under wood floors but they are not as efficient. A stained, concrete floor, cut to look like huge tiles is a popular floor these days and in floor heat works great in that. You ask if there is a way to reduce the cost. I think a hydronic system would work in an insulated adobe floor, but I have never done that. In a very small home you might be able to heat the water with something less than a high tech boiler. You would probably sacrifice some efficiency but that would be compensated for by the small size of the area. Pump it with a low voltage pv system. Heat the water with a homemade coil on some kind of firebox. I grew up in a home where the only hot water was heated with the kitchen wood-fired cook stove, so........

Radiators, either old fashioned cast iron, or modern steel are less costly and also good systems, with the advantage of being more accessible and repairable if there is a problem.

In my current home I have a masonry wood heater. Quite similar in many ways to a rmh. Efficient and burns clean. A very old way of heating in N. Europe.

And lastly, the massive wall or floor that collects solar energy on the right site in the right climate is wonderful.

Okay. Now energy source. Aside from the passive solar energy that heats a mass, which then radiates heat to us, all of the systems require that we actively bring energy from "fuel." Wood. Oil. Propane. Natural Gas. You can buy a boiler that uses any of these and some can switch from oil to wood if needed. The CO2 emission issue, sequestration of carbon, footprint, air quality, etc etc are well known problems and too complex to discuss here. It would be wonderful if all of us had the perfect climate and site where we could use only solar energy to heat our homes. The energy source we choose is impacted by budget, location, size of house, personality, etc., so it is difficult if not impossible to answer your question. About all I can do is rank my choices.

1. Passive solar with a mass wall or floor seems like the best we can do but it has the downside of using concrete, or the extreme labor cost of stone or masonry, and some real restrictions on design and size of house. Depending upon the quality of the solar exposure I would back up this with the most convenient thing I could get, knowing I would rarely use it. Electricity. Propane. Gas. Oil. Wood.
2. Hydronic floor. Needs fuel, none of which is perfect. High cost.
3. Radiators. ditto Can be included in a remodel.
Note: Hydronic systems need a pump but fortunately they are small and can be integrated into a pv system. The can also be powered with a battery backup for grid failures.
4. A masonry heater. High cost. Wood only.
5. RMH low cost. Wood only. Probably the cobb "bench" for mass which is a questionable design feature. Maybe in floor. Tiny wood. Temperamental but hard to beat on a cost basis.
6. High quality cast iron wood stove. Steep heating curve, and I've never seen one that doesn't stream pollutants off the door when you open it to add wood. If you have asthma, stay away from these things.
7. Steel wood stove. Steeper heating curve. Same problem with effluents.
8. Open fireplace. (Often an actual energy loser. Radiates for awhile and then sucks warm air out of your house!) Actually a terrible choice. I know there are fireplace advocates out there, but I think they are full of hot air they didn't get from the fireplace.

I have sort of gravitated into the wood world with this list but it should be mentioned that there are many many other options. We can put electric heating elements almost anywhere. The floor. The walls. The ceiling. All invisible. Electricity is an overall inefficient way to heat things but if your costs are low enough per kwh these systems can look pretty good. Here, where I live in NW WA electric heat in a well-built house competes quite well with wood, unless you have your own free wood. Also, when the grid goes down you have no heat. There are all kinds of gas or oil heaters of course and then there's the pellet stove which requires electricity, etc. etc.

It is ironic that you are asking me these questions because I am currently planning two houses, both for us. One is in the city and the other is in the forest. The city house will have a radiant floor, or if we buy an existing house, we'll retrofit it for radiators. We'll use natural gas to fuel the boiler ( yes, I know about fracking ) and also get domestic hot water. We'll build (or buy) small and concentrate on the structure's insulation and air infiltration control. We will have a cast iron wood stove, just for the aesthetic of the evening fire I love. I've been cutting and splitting wood for decades and I'm getting tired. In the forest, we'll build even smaller and heat with an attempt at passive solar backed up with cast iron wood heater. We'll also have a pv system, probably 12 volt, out there.

So you see, I struggle with these choices for us.

And out beyond all those considerations is the big picture re carbon, sustainability, etc. Everyone should have a look at an Odum Energy Analysis before they get too excited about being environmentally correct. This is why I appreciate some of the critics of each of the alternative building systems. We can be environmentally perfect in one respect and not in many others. As an example, I have a son who lives and works in Seattle. He walks to work and does not own a car. He has no intention to ever own one. Doesn't like to drive. Rides the bus. Rides his bicycle. His carbon footprint is a tiny fraction of mine. I drive a truck and burn trees to heat my home. I'm doing some good stuff ecologically, managing a forest project back into a diverse, mature, food forest( I hope), experimenting with native edibles, etc., but who knows if I will ever have the "carbon account" equal to my son who does not drive.??

Wouldn't it be great if things were simple, like they were back when I was a sophomore in college? (1954)

Great post, Dave. So good, I don't have anymore questions to bug you with.